The present invention relates to a negative pressure booster equipment to be used for brake booster and the like, and particularly to a negative pressure booster equipment, in which two or more power pistons are installed in tandem form.
Conventionally, in order to obtain bigger braking power by smaller depressing force on the brake pedal in the brake booster using negative pressure, a tandem type brake booster is used, in which two power pistons are arranged in tandem as described, for example, in the Japanese Provisional Utility Model Publication No. 63-53860.
In such tandem type brake booster, when the brake pedal is depressed for braking, the control valve is switched over, and the air with atmospheric pressure is introduced into two variable pressure chambers divided by two power pistons. By the action of the atmospheric pressure of the introduced air, two pistons are operated. By the action of these power pistons, the master cylinder is operated to generate the braking fluid pressure, and the braking operation is performed. In this case, the piston of the master cylinder is operated by two power pistons, and the maximum braking fluid pressure thus generated is higher than that of the brake booster equipment operated by a single power piston.
However, in such tandem type brake booster, two variable pressure chambers are installed, into which the air with atmospheric pressure is introduced during braking, and the total volume of these chambers is for larger than that of the variable pressure chambers in the brake booster using a single power piston. If the control valve of the same size as the control valve in the brake booster using a single power piston is used, much time is required until the pressure in two variable pressure chambers reaches the pressure to operate two power pistons, and this causes the delay of power piston operation. As shown by the broken line c in FIG. 4, the rising of the output of the brake booster in the initial stage of operation is delayed. Thus, the conventional tandem type brake booster has lower responsiveness than the brake booster using a single power piston.
To increase the responsiveness of the brake booster, various attempts are made to increase the opening of the control valve, or to enlarge the area of passage of the air flowing into the variable pressure chambers. However, when the opening of the control valve is simply increased, the valve stroke must be set at the higher value, and this results in the increase of the loss stroke of the booster. Also, simple enlargement of the area of air passage means the increase of the dimension of the booster in radial direction. Thus, the new problems arise with regard to the performance characteristics and the size of the booster in any of these cases.